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Date: Tue, 18 Aug 2020 23:50:23 +0300
From: Alexander Popov <>
To: Andrey Konovalov <>,
 Dmitry Vyukov <>, Alexander Potapenko <>,
 Andrey Ryabinin <>,
 kasan-dev <>
Cc: Kees Cook <>, Jann Horn <>,
 Will Deacon <>, Christoph Lameter <>,
 Pekka Enberg <>, David Rientjes <>,
 Joonsoo Kim <>,
 Andrew Morton <>,
 Masahiro Yamada <>,
 Masami Hiramatsu <>, Steven Rostedt
 <>, Peter Zijlstra <>,
 Krzysztof Kozlowski <>,
 Patrick Bellasi <>,
 David Howells <>, Eric Biederman <>,
 Johannes Weiner <>, Laura Abbott <>,
 Arnd Bergmann <>,
 Greg Kroah-Hartman <>,
 Linux Memory Management List <>,, LKML <>,
Subject: Re: [PATCH RFC 1/2] mm: Extract SLAB_QUARANTINE from KASAN

On 18.08.2020 18:45, Andrey Konovalov wrote:
> On Mon, Aug 17, 2020 at 7:32 PM Alexander Popov <> wrote:
>> On 15.08.2020 19:52, Kees Cook wrote:
>>> On Thu, Aug 13, 2020 at 06:19:21PM +0300, Alexander Popov wrote:
>>>> Heap spraying is an exploitation technique that aims to put controlled
>>>> bytes at a predetermined memory location on the heap. Heap spraying for
>>>> exploiting use-after-free in the Linux kernel relies on the fact that on
>>>> kmalloc(), the slab allocator returns the address of the memory that was
>>>> recently freed. Allocating a kernel object with the same size and
>>>> controlled contents allows overwriting the vulnerable freed object.
>>>> Let's extract slab freelist quarantine from KASAN functionality and
>>>> call it CONFIG_SLAB_QUARANTINE. This feature breaks widespread heap
>>>> spraying technique used for exploiting use-after-free vulnerabilities
>>>> in the kernel code.
>>>> If this feature is enabled, freed allocations are stored in the quarantine
>>>> and can't be instantly reallocated and overwritten by the exploit
>>>> performing heap spraying.
>>> It may be worth clarifying that this is specifically only direct UAF and
>>> doesn't help with spray-and-overflow-into-a-neighboring-object attacks
>>> (i.e. both tend to use sprays, but the former doesn't depend on a write
>>> overflow).
>> Andrey Konovalov wrote:
>>> If quarantine is to be used without the rest of KASAN, I'd prefer for
>>> it to be separated from KASAN completely: move to e.g. mm/quarantine.c
>>> and don't mention KASAN in function/config names.
>> Hmm, making quarantine completely separate from KASAN would bring troubles.
>> Currently, in many special places the allocator calls KASAN handlers:
>>   kasan_cache_create()
>>   kasan_slab_free()
>>   kasan_kmalloc_large()
>>   kasan_krealloc()
>>   kasan_slab_alloc()
>>   kasan_kmalloc()
>>   kasan_cache_shrink()
>>   kasan_cache_shutdown()
>>   and some others.
>> These functions do a lot of interesting things and also work with the quarantine
>> using these helpers:
>>   quarantine_put()
>>   quarantine_reduce()
>>   quarantine_remove_cache()
>> Making quarantine completely separate from KASAN would require to move some
>> internal logic of these KASAN handlers to allocator code.
> It doesn't look like there's quite a lot of KASAN-specific logic there.
> All those quarantine_*() calls are either at the beginning or at the
> end of some kasan annotations, so it should be quite easy to move
> those out. E.g. quarantine_reduce() can be moved together with the
> gfpflags_allow_blocking(flags) check and put before kasan_kmalloc()
> calls (or maybe also into some other places?), quarantine_put() can be
> put after kasan_slab_free(), etc.
>> In this patch I used another approach, that doesn't require changing the API
>> between allocators and KASAN. I added linux/mm/kasan/slab_quarantine.c with slim
>> KASAN handlers that implement the minimal functionality needed for quarantine.
>> Do you think that it's a bad solution?
> This solution doesn't look clean. Here you provide a second KASAN
> runtime implementation, parallel to the original one, which only does
> quarantine. It seems much cleaner to put quarantine logic into a
> separate module, which can be either used independently, or together
> with KASAN built on top of it.

That sounds reasonable, I agree. Thanks, Andrey.
Added to TODO list.

At first I'm going to focus on exploring security properties of the quarantine.
And then I'll do the refactoring that you and Kees propose.

Best regards,

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